Why do my two cats suddenly hate each other?

Usually a triggering event reset their social pattern. Common triggers: one cat coming back from a vet visit smelling different (the other doesn’t recognize her, attacks the “intruder”), a new pet or person in the home, a household renovation, an outside cat visible through the window. Cornell flags redirected aggression and inter-cat conflict as common scenarios where a stressor changes the dynamic.[1] The fix is often a partial re-introduction protocol: separate them for a few days, scent swap, then gradual re-introduction the way you’d introduce new cats. Pheromone diffusers can be a layered tool on top of the protocol.

When to call a vet: if either cat is being injured, refusing food, or hiding for days, slow down and consider a behavior consultation. Aggression that doesn’t respond to reintroduction protocols sometimes has a medical cause.